The present invention relates to a method and an apparatus for controlling the idling speed of an internal combustion engine by controlling the intake air flow rate.
A known system for controlling the idling speed consists of controlling, by a closed-loop, the closed position of a throttle valve or the opening degree of a flow-control valve in a bypass intake passage which is provided in parallel with an intake passage that accommodates the throttle valve, by using a step motor depending upon the rotational speed of the engine during idling. In such a system, the open-loop control, not the closed-loop control, of the idling rotational speed is executed during cranking by applying a drive signal having a predetermined constant value to the step motor.
However, during cranking, since a large current flows to the starter motor, the battery voltage is greatly lowered, causing the step motor to fail to actuate or to incorrectly actuate. In other words, during cranking, there often occurs a step-out operation of the step motor. At a low atmospheric temperature, since the battery voltage is greatly lowered, the step-out operation of the step motor is more likely to occur. The above step-out operation causes a difference between the actual rotating position of the step motor and the rotating position as recognized by the control circuit. As a result, the intake air flow rate, which determines the idling rotational speed, cannot be correctly controlled either during cranking or after cranking.